1. Field of Invention
The invention relates to methods and systems that compensate for polarization mode dispersion.
2. Description of Related Art
As data rates transmitted over optical conduits such as long single-mode optical fibers increase, the effects of polarization mode dispersion (PMD) become increasingly important. PMD is a form of signal distortion and can be caused by subtle physical defects in an optical fiber giving rise to birefringence of the optical fibers. The effects of this phenomenon are often categorized into first-order PMD effects and higher-order PMD effects.
First-order PMD refers to the time dispersal of various components of an optical signal that is essentially constant for all frequencies in a narrow band of optical frequencies. First-order PMD is equivalent to splitting a transmitted optical signal into two orthogonal polarization components along a birefringence axis of an optical fiber, and delaying one of the polarization components relative to the other, to produce multiple images of the optical signal.
Second-order, or higher-order, PMD refers to temporal dispersal that, unlike first-order PMD, varies as a function of frequency and can result when the axis of birefringence varies along the length of an optical fiber. While the use of a PMD compensator can mitigate the deleterious effects of PMD, the vast majority of PMD compensators are designed for first-order PMD only. Accordingly, there is a need for new technology to provide better PMD compensation.
The invention provides methods and systems for PMD compensation using an economical number of components. The technique includes passing an optical signal through a frequency-dependent polarization rotator, then through a first-order PMD compensation device, then again through a frequency-dependent polarization rotator, which has a fixed relation to the first frequency-dependent polarization mentioned above. This combination of elements allows compensation for certain high-order features of PMD. Further economy can be gained by applying economical optical design concepts such as selectively reusing common optical components in the compensation model.